B-cell malignancies comprise the major subtype of lymphomas today with over 100,000 new cases per year. The vast majority of patients are not curable despite the apparent sensitivity of these diseases to a number of drugs and biologic agents commonly in use. A characteristic of many B-cell lymphomas and leukemias is that chemotherapy and/or biologic based responses are readily obtainable, but cures are more difficult. B-cell lymphoma can be a very aggressive disease where many of the patients do not respond to conventional treatment. It is apparent that residual clones of neoplastic cells remain after log cell kill with chemotherapy and/or biologic therapies. The ability to cure these diseases will be dependent on the success of eradication of all tumor cells, especially tumor stem cells. Anti-CD20 antibodies such as rituximab, ofatumumab, obinutuzumab, and tositumomab, have also been used to treat B-cell derived malignancies as single agents, as potentiators of chemotherapy, as maintenance therapy and as vehicles to deliver radioisotopes/drugs. These antibodies bind to CD20, a restricted B-cell differentiation antigen expressed only by normal and malignant B-cells. Because of the expression of CD20 antigen on both normal and malignant B-cells, anti-CD20 antibodies can also lead to the destruction of a portion of normal B-cells, the long term consequences of which are unknown. (See, Smith M R, Oncogene 22:7359-7368 (2003); Jacobs S A, et al., Expert Opin Bio Ther 7:1749-1762, (2007)).
In contrast to CD20, the B-cell Receptor Complex (BCRC) is the central differentiation signaling element of the B-cell arm of the immune system and this molecule is expressed on the surface of all B-cell malignancies. The BCRC comprises a cell-surface membrane bound Ig (mIg) (such as mIgM, mIgG, mIgA, mIgE and mIgD) and a closely associated co-signaling molecule CD79αβ. Previous strategies to target the BCRC molecules in B-cell malignancies have focused on the unique CDR sequences specific for each monoclonal tumor. (See, Miller R A, et al., N Engl J Med 306:517, (1982); Levy R, et al., J Natl Cancer Inst Monographs 10:61 (1990); Davis T A, et al., Blood 92:1184-1190 (1998)). However, as a consequence of the uniqueness of each CDR, this approach necessitated the generation of a specific drug for each patient, which proved not to be feasible in the clinic. These early clinical studies targeting the BCRC did, however, demonstrate anti-tumor activity.
The B-cell Receptor (BCR) initiates a driver pathway in B-cell lymphoma-leukemia. One strategy has been to target BCRC associated cytoplasmic molecules such as the Syk tyrosine kinase, a downstream mediator of the BCRC signaling pathway, to inhibit downstream pathway tyrosine kinases. Both vertical and horizontal membrane BCRC interactions render this downstream pathway complex and redundant. As the Syk tyrosine kinase pathway is not restricted to B-cell lineage tissue, its inhibition leads to unwanted immune effects, possible pro-oncogenic effects in breast tissue and other toxicities in non-hematopoietic cells. Further downstream of the BCRC is the Bruton tyrosine kinase (BTK). Bruton tyrosine kinase inhibition has also emerged as a compelling target downstream of the BCR, which is now an approved strategy through the utilization of the drug ibrutinib. The approval of ibrutinib, the first BTK inhibitor demonstrating potent activity, provides compelling evidence of the significance of the BCRC in driving B-cell malignancies. Additional molecular targets have been identified downstream of the BCRC, such as PI3K delta and BCL2, and drugs blocking the activity of these targets are also shown to have significant clinical activity.
As a consequence of the BCR's sequence homology to serum Ig, developing specific anti-membrane Ig therapy was a hurdle. Specific mIgM targeting in vivo was thought not to be feasible, as the drug or biologic would bind to the circulating IgM in blood prior to reaching the cell surface B-cell membrane mIgM. A unique set of sequences previously identified in the membrane-bound Igs, designated proximal domains (PDs), are not expressed in serum Igs. These PDs are Ig class specific and for mIgM constitutes a 13 amino acid peptide. However, attempts to produce anti-mIgM PD antibodies were not successful due to the low immunogenicity of the PD peptide, its hydrophobicity, and the resultant low affinity of the generated antibodies. In contrast, efforts to produce mIgE PD have resulted in several functionally distinct versions. See, e.g., U.S. Pat. No. 8,137,670; 8,404,236; Poggianella M, et al., J Immunol. 177:3597-3605 (2006); Feichter S, et al., J Immunol 5499-5505 (2008).
There is a need for antibodies that have a high level of specificity for B-cell mIgM in order to internalize the receptor, inhibit cell growth, induce apoptosis or deliver drugs, toxins or radioisotopes to these mIgM B-cells, while sparing normal lymphocytes (non-mIgM expressing B-cells) and non-lymphatic tissues from toxicity. Such antibodies can also be used in diagnostics of B-cell lymphomas and B-cell leukemias. These uniquely specific antibodies will allow for the first time the ability to separate membrane IgM from serum IgM by immune-affinity methodology.